Ultracold Quantum Gases Group

Welcome to the ultracold quantum gas research group at Aarhus University!

In our research we investigate the properties of atomic gases at extremely low temperatures. This allows us to understand the fundamental quantum mechanical behaviour of few- and many-particle systems.


Faraday image analysis

Sub-atom shot noise Faraday imaging of ultracold atom clouds

We have developed an imaging technique which can measure the atom number below the atom shot noise level. This work is closely related to our recent work on feedback stabilization of atom numbers. We use Faraday imaging which allows multiple images of the same cloud to be acquired. To describe the expected noise, we have developed a model based on photon shot noise and single atom loss. For clouds containing N~5×106 atoms, a precision more than a factor of two below the atom shot noise level is achieved.

Our manuscript on this work has been published in Journal of Physics B as part of a special issue on addressing quantum many-body problems with cold atoms and molecules.

Read the manuscript in Jour. Phys. B or on arXiv!


Three-body recombination in KRb mixtures displays a surprising lack of Efimov physics

Absence of observable Efimov resonances in ultracold KRb mixtures

In previous experiments with ultracold mixtures of potassium and rubidium, an unexpected non-universal behavior of Efimov resonances was observed. We have measured the scattering length dependent three-body recombination coefficient in ultracold heteronuclear mixtures of 39K-87Rb and 41K-87Rb and do not observe any signatures of Efimov resonances. This reestablishes universality of the three-body parameter across isotopic mixtures.

The article is published in Physical Review Letters and available on arXiv.


Coupled interferometer states in a Rb BEC

Improvement of an Atomic Clock using Squeezed Vacuum

Since the pioneering work of Ramsey, atom interferometers are employed for precision metrology. In a classical interferometer, atoms are prepared in one of the two input states, whereas the second one is left empty. In this case, the vacuum noise restricts the precision of the interferometer to the standard quantum limit (SQL). We have experimentally demonstrated a novel clock configuration that surpasses the SQL by squeezing the vacuum in the empty input state. As such, 0.75 atoms improve the clock signal of 10,000 atoms!

Published in Physical Review Letters and available on arXiv.



Goodbye Andrew!

Andrew got an industry job in New Zealand and is thus leaving the group. Throughout the last years, he has been a vital part of the Lattice lab, conducting remarkable research. Besides his work in the Lattice lab, his expertise has also been a major benefit for the rest of the Ultracold Quantum Gases Group. We wish him and his family the best in New Zealand!


Figure selected for Phys. Rev. A Kaleidoscope

A figure from our recent paper on phase separation and dynamics of two-component Bose-Einstein condensates was selected to be on display as part of the Phys. Rev. A Kaleidoscope!

Extra credit goes out to Kean Loon Lee who was main author on the paper.

Read the paper here or on arxiv




The Faraday imaging system and the FPGA at work

Choose the Number of Atoms in Your Cloud: Preparation of Ultracold Atom Clouds at the Shot Noise Level

Experiments with ultracold atoms inherently suffer from shot-to-shot atom number fluctuations which limit the precision. The UQGG Lattice team have demonstrated a technique for preparing a large cloud of a specific number of atoms with unprecedented low uncertainty. The usual atom number fluctuation of about 10% are reduced to below 0.1%!

During the experimental procedure, a series of non-destructive Faraday images probe the number of atoms in the cloud. A field programmable gate array provides online data analysis and performs feedback by removing atoms from the cloud until the desired number of atoms are reached. Finally, a second series of Faraday images confirm the number of atoms remaining in the cloud.

By creating similar atom clouds reproducibly, this newly developed technique can potientially improve the performance of atomic clocks and other high-precision measurements or simply just reduce the number of hours the typical graduate student has to spend in the lab to obtain data of sufficiently high quality.

The results have been published in Physical Review Letters as an Editors' Suggestion. Additionally, the work is featured in Physics, where a Focus article was written. The article can be found on arXiv as well.


Theis embracing a dear deer

Goodbye Miroslav! Welcome Magnus and Mikkel! Congratulations Theis!

Recently, Miroslav Gajdacz got a job at OFS Denmark, which develops optical fibres, and he thus left the group. He has done great research in the Lattice lab, where his main contribution was the implementation of Faraday imaging and the use of this to prepare ultracold atom clouds at the shot noise limit. Additionally, he has published several theory papers on atomtronics and the quantum speed limit. Based on his research, he obtained his PhD last fall and has since then continued his studies as a Posdoc. We wish him the best of luck in his future endeavours!

The next generation of PhD students in the UQGG has however arrived! Mikkel Berg Christensen has started in the Lattice lab. The recently developed precise production of ultracold samples will allow him to aid in exploring new frontiers of quantum gases. Additionally, Magnus Graf Skou has started in the MIX lab. The recent observation of polarons in a Bose-Einstein condensate has opened up for studies of quantum impurities in regimes never before realized.

Finally, Theis recently obtained his masters degree! He was a great asset for the group throughout the last year. His main contribution was made in the MIX lab where he aided to observe the Bose polaron. He will continue to work in the group as a research assistant.


The polaron in a Bose-Einstein condensate

Observation of Attractive and Repulsive Polarons in a Bose-Einstein Condensate

Mobile impurity particles interacting with a bosonic quantum environment play a central role in our understanding of nature and are fundamental for several important technologies such as organic electronics. It is therefore highly desirable to study impurity physics systematically and from a broad perspective as offered by cold atomic gases.

We present the experimental realization of long-lived impurity atoms in an atomic Bose-Einstein condensate. The energy of the impurity is measured and we find excellent agreement with theories that incorporate three-body correlations, both in the weak-coupling limits and across unitarity. For both strong repulsive and strong attractive interactions, our experimental results demonstrate the existence of a polaron quasiparticle.

The manuscript has been published in Physical Review Letters as an Editors' Suggestion, back-to-back with results from the group of Eric Cornell and Deborah Jin at JILA. Additionally, it can be found on arXiv.

Our results are also featured in several news outlets which appeal to a broader audience:

Physics Viewpoint: Bose Polarons that Strongly Interact

Quasipartikler som klumper i kold kvantesuppe (danish)


The difference between single particle and collective spontaneous emission.

Semi-classical dynamics of superradiant Rayleigh scattering in a Bose–Einstein condensate

Due to its coherence properties and high optical depth, a Bose–Einstein condensate [BEC] provides an ideal setting to investigate collective atom-light interactions. Superradiant light scattering in a BEC is a fascinating example of such an interaction. It is an analogous process to Dicke superradiance, in which an electronically inverted sample decays collectively, leading to the emission of one or more light pulses in a well-defined direction. Through time-resolved measurements of the superradiant light pulses emitted by an end-pumped BEC, we study the close connection of superradiant light scattering with Dicke superradiance. A 1D model of the system yields good agreement with the experimental data and shows that the dynamics result from the structures that build up in the light and matter-wave fields along the BEC. This paves the way for exploiting the atom–photon correlations generated by the superradiance.

You can read the article in a special issue of Journal of Modern Optics and on arXiv. (07/2016)

Evaporation and probing procedure to determine the BEC transition

Measurement-enhanced determination of BEC phase diagrams

Shot-to-shot fluctuations plague many experiments within the field of quantum gases. We have demonstrated how dispersive atom number measurements during evaporative cooling can be used for enhanced determination of the non-linear parameter dependence of the transition to a Bose-Einstein condensate (BEC).

You can read the first experimental paper from the HiRes lab on arXiv. (07/2016)

Phases of two interacting BECs

Phase Separation and Dynamics of two-component Bose-Einstein condensates

The miscibility of two interacting quantum systems is an important testing ground for the understanding of complex quantum systems. Two-component Bose-Einstein condensates enable the investigation of this scenario in a particularly well controlled setting. In a homogeneous system, the transition between mixed and separated phases is characterised by a 'miscibility parameter'. In this theoretical analysis we have shown that this parameter is no longer the optimal one for trapped gases, for which the location of the phase boundary depends critically on atom numbers.

The manuscript has been published in Physical Review A and is also available on arXiv. (07/2016)

Research receives attention in popular media

Recently, an international collaboration including Jan Arlt published an article in Nature Communications: Satisfying the Einstein–Podolsky–Rosen criterion with massive particles

Two popular articles were written in danish based on this research:

Den havde Einstein ikke set komme (Einstein didn't see that coming)

Ny fysisk metode bruges til at lave afsindigt præcise målinger (New method in physics allows incredibly precise measurements)


Jan Arlt

New grant: Experiments with Quantum Test Beds

Jan Arlt has received a grant from VILLUM FONDEN valued at DKK 4.7 million. View more details here. (01/2016)


The Villum Foundation